The present invention relates to the hydraulic circuit for providing fluid to a vehicle's steering and auxiliary functions and, more particularly, relates to such hydraulic circuits giving steering priority over auxiliary functions and having an accumulator for providing emergency steering capability.
Industrial vehicles such as skidders, loaders and scrapers, or the like, are provided with hydraulically fed steering systems which make it possible for large vehicles to be maneuvered with relative ease during all operations. Such vehicles are also provided with hydraulically fed auxiliary function capabilities to operate such things as grapples, loader buckets and scraper blades. The auxiliary functions share hydraulic fluid with the steering system. Conventionally, hydraulic fluid is passed through a priority valve which branches fluid to both the steering system and the auxiliary functions giving priority to the steering system. That is, the fluid requirements of the steering system have preemption over the fluid requirements of the auxiliary function. Emergency steering capability is provided by an accumulator which is in fluid communication with a secondary valve which ports accumulator contained fluid to the steering system upon sensing the last of a substantial degree of fluid fed pressure to the steering system.
Conventionally, the priority valve includes a valve spool acted upon by fluid pressure to overcome a spring and assume a first spool position. The first position allows the priority valve to port fluid to both the main steering valve of the steering system and to the auxiliary functions. When the fluid pressure drops below a predetermined amount occasioned by a fluctuation in hydraulic circuit pressure, the spring force on the valve spool shifts the valve spool to a second position porting all incoming fluid to the main steering valve.
Large vehicles utilize steering systems which require relatively high fluid pressure on the order of 1500 to 2000 psi, in contrast, the fluid pressure requirements of auxiliary functions can be only a few hundred psi under certain conditions. As a result, when the vehicle is using an auxiliary function and is not being steered, the conventional priority valve, which is pressure sensitive, experiences high pressures necessary to overcome the spring force on the valve spool. The absence of the production of work in light of high spool valve pressures results in the production of excessive heat necessitating the inclusion of a heat exchanger in the hydraulic circuit.